In order to limit the acoustic frequencies reproduced by a speaker within a desired range, there has been a universal practice to insert a low-pass filter or a bandpass filter comprising an electronic circuit into the input stage of the speaker, but there has been also devised and demonstrated a speaker of the type in which a mechanical filter is incorporated into the speaker so that the range of reproduced acoustic frequencies may be limited and the characteristics substantially similar to those attained by the insertion of a low-pass filter or a bandpass filter comprising an electronic circuit may be realized. In the latter type speakers, used in general as a mechanical filter is a center holder or retainer made of a phenol-impregnated and corrugated fabric and interposed between a voice coil bobbin and a diaphragm.
When the center holder or retainer has been used for a long time, it is subjected to fatigue so that phenol is broken and consequently the compliance is increased. As a result, the filter characteristics are varied and subsequently the range of reproduced acoustic frequencies is changed.
There has been also devised and demonstrated a speaker of the type which is mounted on a partition wall disposed within a speaker box or cabinet; an air-tight chamber is defined between the speaker box or cabinet and a front plate of the speaker box; and a passive cone is mounted on the front plate, whereby the vibrations of a diaphragm cause the vibrations of the air trapped in the air-tight chamber which in turn cause the passive cone to drive. In this case, the air-tight chamber acts as a mechanical filter, so that a desired range of reproduced acoustic frequencies may be determined by suitably determining the volume of the air-tight chamber.
However, the box of the speaker system of the type described above becomes large in size and the partition plate or wall and the passive cone must be provided in addition to the speaker. As a consequence, there arises the problem that the costs increase.